1. Field of the Invention
The present invention deals with heated sealing bars commonly used to fuse layers of plastic film in bag making machinery, including those seal bars which simultaneously heat seal and sever portions of the web.
2. Prior Art
Heat sealing bars commonly used in bag making machinery commonly consist of two basic assemblies. The first is a structural beam extending across the width of the bag machine and mounted to the mechanism used to move the seal bar towards and away from the plastic web, while the second is a shaped bar used to contact the web and which includes one or more heating elements, temperature measuring elements, and common structural fasteners. The heated sealing bar is desirably mounted to the structural beam in such a manner that the heat flow path from the seal bar to the beam is small and so that heating or repeated heating of the seal bar does not result in distortion of the sealing bar. Although minimum cross sectional heat flow areas have been attained, freedom from distortion has remained a chronic problem.
U.S. Pat. No. 3,779,838 discloses and claims a seal bar structure which consists of an upper half and a lower half sandwiched around the heating elements. The upper half of the seal bar is discontinuous and is resiliently mounted to the continuous lower half, with the heating element between the upper and lower bar halves. Alternate sections of the upper half of the seal bar are mounted to the support beam or solely to the lower half of the seal bar. The heating element is relatively free to expand, thereby eliminating warping stresses and maintaining the bar straight.
U.S. Pat. No. 4,449,962 discloses and claims a seal bar structure intended to allow relatively free expansion of the heated seal bar structure with respect to the supporting beam, while further providing the ability to deform the bar to compensate for any distortion which does occur. The heated seal bar assembly of the patent, at least as far as the issue of deformation is concerned, is of typical configuration, i.e. continuous upper and lower halves sandwiched about the heating element. What is new, however, is the method of connecting the seal bar to the supporting beam. The supporting beam consists of upper and lower flanges connected by diagonal bracing. The seal bar is attached to the support beam by a series of long threaded rods which pass through the upper and lower flanges of the support beam and into the upper half of the seal bar. Accommodation for lengthwise expansion is provided by elongating the apertures through which the long threaded rods pass in the longitudinal direction of the beam. A series of Belville washers are placed between the nut on the threaded rod and the upper surface of the upper flange to bias the seal bar towards the beam. Two threaded bores in the lower flange are arranged diagonally about each elongated aperture. Screws are passed through these threaded bars into contact with the upper surface of the seal bar. Judicious tightening and loosening of all the bolts allow correction of vertical plane distortion of the seal bar.
A thermo compression bonding tool is disclosed and claimed in U.S. Pat. No. 4,431,474. The bonding rail, which is of similar construction to a seal bar, i.e. a relatively long shaped metal section about an electric heating element, is biased against a plurality of alignment ribs on a support element. The lower surfaces of the alignment ribs define the plane against which the upper surface of the bonding rail rests. Near the middle of the bonding rail the bias means is pin connected to the rail; the rail is free to expand longitudinally along the plane defined by the alignment ribs.